As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed, and has been practically employed. The radiation image recording and reproducing method employs a stimulable phosphor sheet (i.e., radiation image storage panel) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The sheet thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the stimulable phosphor sheet can be repeatedly employed.
In the above method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resource and economic efficiency because the stimulable phosphor sheet can be repeatedly used while the radiographic film is consumed for each radiographic process in the conventional radiography.
The stimulable phosphor sheet has a basic structure comprising a support and a stimulable phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. The phosphor layer usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, the layer comprising agglomerated phosphor soaked with a polymer is also known. In any types of phosphor layers, the stimulable phosphor emits stimulated emission when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the stimulable phosphor sheet in an amount proportional to the applied radiation dose, and a radiation image of the object is produced in the sheet in the form of a latent image (i.e., radiation energy-stored image). The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the phosphor sheet with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical shock.
The radiation image recording and reproducing method can be performed by means of an all-in-one type apparatus comprising recording means (by which a radiation image is recorded on the phosphor sheet), reading means (by which the image recorded in the phosphor sheet is read by the steps of exciting the stimulable phosphor with stimulating rays to release stimulated emission and photoelectrically detecting the emission), erasing means (by which the radiation image remaining in the phosphor sheet is erased with erasing light), and conveying systems connecting each means for conveying the phosphor sheet. The above-mentioned means may be separated into two apparatuses, namely a recording apparatus comprising the recording means and a reading apparatus comprising the reading means and the erasing means.
The radiation image recorded in the phosphor sheet is generally read by applying stimulating rays onto one surface side (the phosphor layer side) of the phosphor sheet and collecting light emitted by the phosphor particles by means of a light-collecting means from the same side. A system for reading the image from one side of the sheet in this manner is referred to as "single-side reading system". However, there is a case that the light emitted by the phosphor particles should be collected on both sides of the phosphor sheet. For instance, there is a case that it is desired to collect the emitted light as much as possible. There is also a case that the radiation image recorded in the phosphor layer varies along the depth of the layer, and that the variation should be detected. A system for reading the image from both sides of the sheet is referred to as "double-side reading system". Japanese Patent Provisional Publication No. 55(1980)-87970, for example, discloses the double-side reading system.
Each of Japanese Patent Provisional Publications No. 7(1995)-174897, 7(1995)-287100 and 8(1996)-62750 discloses a process and an apparatus for applying erasing light onto both surfaces of the sheet after reading the recorded image according to the double-side reading system, so as to release the radiation energy which remains after the reading procedure.
The radiation image recording and reproducing method has various advantages described hereinbefore, but it is still desired for the method to have a higher sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.).
The image quality is generally impaired by noises caused by a residual radiation image and other stored radiation energy given by environmental radiation or radioactive isotopes contaminated into the phosphor sheet. For obviating these noises, the erasing procedure is required. Naturally, it is desired that the erasing procedure be carried out as effectively as possible. For example, it is desired that the time and the energy for erasing is shorter and lower, respectively. In addition, it is desired that the ratio of amount of stimulated emission after erasing to that before erasing (hereinafter, this ratio is referred to as "erasing value") is as small as possible.